POWER GENERATION AND TRANSMISSION 11 A diagonal stay or two will then be necessary and the heating surface will be reduced. The aggregate cross-sectional tube area should be one-ninth to one-tenth the grate area for hard coal and one-sixth to one-seventh for soft coal. With horizontal and vertical pitches equal to p in., the number of tubes per square foot of tubed head area is 144 -5- p2. The distance from the center line of the boiler to the center of the top row of tubes may vary from 6J^ to 10K in. for shell diameters ranging from 48 to 72 in. The following pitches are typical: Tube diameter, inches................. 3 3J^ 4 Horizontal pitch, inches................4}^ to 4K 5 to 5K 6 Vertical pitch, inches.................. 4 4J^ 5 The maximum practicable number of tubes is given by Parsons, as below: Tube thicknesses for pressures up to 175 Ib. range from No. 13 for sizes under 2K in. to No. 9 for 5 in. (B.w.g.). Water-tube Boilers.—The Bab-cock & Wilcox stationary boiler is usually built of 4-in. tubes expanded into vertical or slightly inclined headers which communicate with Overhead drums and a small transverse mud drum in the rear. Overhead drums are most commonly' longitudinal, 24 to 42 in. diameter. The tubes incline downward from front to rear. If inclined headers are used, the header plane is at right angles with the tubes. For low pressures, headers may be of cast iron. Steel forgings are used for pressures above 160 Ib. In the headers, opposite each tube end, is a hand-hole with inside plate and ball-head bolt. The handhole joint is either ground or provided with a thin gasket. The gases strike the tube banks transversely and usually cross them three times (three pass boiler). Heat extraction is apt to be most thorough when the tube bank is high rather than wide. This means high efficiency, a narrow boiler and low first cost of installation: but may give an insufficiently wide grate for some coals. The Heine boiler has the gas circulation mainly parallel with the tubes, and has flat staybolted pressure surfaces forming headers or water boxes into one side of which the tubes are expanded. A particularly vigorous water circulation probably results from the use of these water boxes. A handhole is placed opposite each tube end. TUBE THICKNESSES (B.w.G.) FOR WATER-TUBE BOILERS Shell Maximum number of tubes diameter, inches Diameter inches : 3 3H 4 60 84 62 48 66 104 86 64 72 126 98 78 78 160 124 100 Diameter, inches Under 3 Under 4 Under 5 5 For 165 Ib pressure 12 11 10 q A S M E For 235 Ib. pressure . . . 11 10 9 8 Code For 285 Ib. pressure ........... 10 9 8 For 400 Ib. pressure ........... 9 8 7 for 4 in. For 210 Ib. pressure ........... 10 for 4 in. Babco ck & For 260 Ib. pressure ........... 9 for 4 in. Wilcox Practice NOTE: All connecting tubes (headers to drums) are No. 9 in Babcock & Wilcox boilers.